This invention relates generally to systems for locating a position of a source of a detectable emittance, and more particularly, to a system which is particularly adapted for locating the position of a scintillation event in single photon emission computer tomography (SPECT).
The estimation of a position of an event or other interaction which affects a plurality of sensors has received considerable attention in a variety of disciplines. Of particular interest is the estimation of the position of a scintillation event in a radiation detector, particularly of the type used in single photon emission computer tomography. Most commonly, SPECT systems utilize detectors which are comprised of arrays of small detector elements, such as photomultiplier tubes. In order to maximize the useful field of view, the photomultiplier tubes are arranged in a predetermined configuration, which may be an array having a rectilinear or hexagonal configuration. The essential purpose of such a system, particularly a SPECT configuration as used in nuclear medicine, is to facilitate the imaging of radionuclides distributions in a predetermined region.
The essential elements of a known scintillation camera of the modular type employs a scintillation crystal which is arranged to communicate with a light guide. The light guide is in turn connected to a plurality of photomultiplier tubes which produce respective electrical signals responsive to the number of optical photons which reach each photomultiplier tube as a result of a scintillation event. As is known, the number of optical photons which impinge on a photomultiplier tube result in the production of a multiplicity of photoelectrons in each photomultiplier tube. It is significant to note, however, that almost all of the events incident to scintillation events and their detection must be analyzed statistically. For example, both, the number of photons generated in each scintillation event, as well as the number of photoelectrons produced in each photomultiplier tube, are statistical. These numbers, therefore, fluctuate about a mean value which, with respect to the number of photoelectrons produced in each photomultiplier tube, is a function of the position of the scintillation event. The uncertainties attendant any given event result in difficulties in ascertaining the precise position of the event.
The prior art has thrust at the problem of position estimation from a variety of approaches. One known method utilizes a completely digital system which employs look-up tables stored in memories. It is a problem with the known system that is requires a very large memory for more than two or three sample inputs. Also, the method depends upon the spatial resolution of the position estimation. In situations where four phototubes are employed, this known system could easily require sixteen megabytes of memory to estimate only sixty-four discrete positions.
A further known system utilizes both analog and digital aspects, and employs resistor networks and summing amplifiers. It is a problem with this method that it is inflexible and quite expensive. Again using four phototubes for example, this known system would require sixty-four amplifiers and 256 precision resistors to achieve the same estimation. The resistors may need to be nonstandard values, thereby increasing the cost. Moreover, such components occupy considerable space and must be mounted individually on circuit boards.
It is, therefore, an object of this invention to provide a simple and inexpensive system for estimating the position of a source of an emittance, or an event, such as a scintillation event.
It is another object of this invention to provide a precision estimating system which can be rapidly and easily configured to suit given detector module characteristics, without requiring changing resistors or other circuit board components.
It is a further object of this invention to provide a position estimator wherein no penalty is incurred as a result of dividing more finely the interpolated position values.
It is also an object of this invention to provide a position estimating system which is easily extended to include a greater number of phototubes.
It is additionally an object of this invention to provide a position estimating system which can be extended to perform multidimensional maximum liklihood estimates.
It is still another object of this invention to provide a precision estimating system which can be speeded up simply and with very little increase in cost.
It is still another object of this invention to provide a position estimating system which can use relatively slow and inexpensive components.